This invention relates to micromachined gyros.
A surface micromachined gyro has a planar body (or a number of bodies) suspended with anchors and flexures over and parallel to an underlying substrate. The body is dithered along a dither axis in a plane parallel to the substrate and perpendicular to a sensitive axis that can be in the plane of the body or perpendicular to the body and to the substrate. As is generally known, rotation by the body about the sensitive axis causes the body move along a Coriolis axis, which is mutually orthogonal to the dither axis and the sensitive axis. This motion can be sensed to derive a signal that indicates the angular velocity of the rotation.
Because of mechanical imperfections in the body and in the flexures, a suspended mass will typically not be perfectly parallel to the substrate, and the dither and sensitive will typically not be perfectly orthogonal. Consequently, when the body is dithered, an interference signal, referred to as the quadrature signal, is induced by the dithering motion itself. This quadrature signal, which is unrelated to the rotation to be sensed, interferes with the desired signal relating to the rotation. The quadrature signal (a) is proportional to the acceleration in the dither direction with a constant of proportionality indicative of the mechanical misalignment; (b) has the same frequency as the dither frequency; and (c) is 90xc2x0 out of phase with the dither velocity, unlike the Coriolis signal which is in phase with the velocity. Because of this 90xc2x0 phase difference, the quadrature signal can be partially rejected with a phase-sensitive detector. The effectiveness of such rejection, however, depends on how precise the phase relationships are maintained in the electronics.
In one aspect, the present invention is a micromachined gyro in which there is minimal interference in the output signal caused by the dither signal. The gyro has a first body, suspended over a substrate and dithered along a dither axis, and a second body coupled to the first body and also suspended over the substrate. The first and second bodies are coupled together and anchored to the substrate such that the first body can move along the dither axis but is substantially inhibited from moving along a Coriolis axis (perpendicular to the dither axis) relative to the second body, and the second body is movable with the first body along the Coriolis axis but is substantially inhibited from moving along the dither axis. The coupling between the first body and the second body substantially decouples the dithering movement from the movement along the Coriolis axis in response to rotation about the sensitive axis, thus minimizing the unwanted quadrature signal. One of the first and second bodies preferably surrounds the other; the dithered first body is preferably on the inside and surrounded by the second body, although the first body can surround the second body.
In another aspect, a micromachined device has a first body with fingers interdigitating with fixed drive fingers that cause the first body to dither along a dither axis. At least one conductive member is formed under some, but not all, of the fixed dither drive fingers and is electrically coupled to the drive fingers to keep the first body in the desired vertical plane and to prevent the first body from levitating due to fringe effects.
In yet another aspect, a micromachined device has a movable body suspended over a substrate and at least one stop member positioned near the movable body. The stop member includes a hook portion extending over the movable body such that the stop member limits both lateral movement and vertical movement by the body.
In still another aspect, a micromachined device has a suspended movable body with an outer perimeter portion and at least one cross-piece that defines a number of apertures enclosed by the perimeter portion. The body has fingers extending into the apertures. These fingers can be used either to dither the body or to sense motion of the body.
In another aspect, the micromachined device has an inner body surrounded by an outer body, the outer and inner bodies being inhibited from movement together along one axis by flexures oriented along that one axis. These flexures are connected between the body and an elongated stationary member anchored at a midpoint and with the flexures extending from each end to the body. The elongated member is preferably between the inner and outer bodies
In still another aspect of the invention, a first micromachined structure is positioned near a second micromachined structure, and the first micromachined structure is dithered relative to the second micromachined structure. These first and second structures are connected together with coupling structures designed to minimize stress and to encourage opposite ends of the structure to move together in the direction of dithering toward and away from the second structure. While there are a number of variations of coupling structures that can be used, these include structures that have elongated members extending from ends of the first structure and extending toward the center of the structure along a direction perpendicular to the dithering direction. These elongated members are connected by a short connecting beam that encourages the elongated members to move together in the same direction at the same time, rather than moving in opposite directions. These elongated members are connected to the first structure with perpendicular members that define a pivot point.
Openings can be cut out of the second structure to reduce the combined mass of the first and second structures, while still maintaining stiffness in the structure. In addition to the coupling structures between the first and second structures, the second structure is also anchored to the substrate through plates that are relatively wide compared to the width of the second structure itself. These plates are connected together by perpendicular members that define a pivot point.
The gyro of the present invention minimizes the quadrature signal, and thus is very accurate compared to prior gyro designs. The accuracy due to the structure of the gyro obviates the need for complex electronics, and also allows the device to be packaged under ambient conditions. The stop members, the conductive members on the substrate, use of multiple apertures with inwardly extending fingers, and use of a centrally anchored stationary member for supporting flexures along an axis of inhibited movement all improve performance and reliability of a micromachined device in general and a gyro in particular. Other features and advantages will become apparent from the following detailed description, the drawings, and the claims.